1. Field of the Invention
The present invention relates to an air-fuel ratio control device of an internal combustion engine.
2. Description of the Related Art
In known engines, when the engine is in the cruising state, the air-fuel mixture is made lean, and when the engine is accelerated, the air-fuel ratio is brought to the stoichiometric air-fuel ratio. When the cruising state of the engine is started after the engine has been accelerated, the air-fuel mixture is made lean again (Japanese Unexamined Patent Publication Nos. 60-13936 and 63-129140). In the engine disclosed in Japanese Unexamined Patent Publication No. 60-13936, when a rate of change in the degree of opening of the throttle valve becomes smaller than a fixed negative value after the acceleration of the engine is completed, the air-fuel mixture is changed from that for the stoichiometric air-fuel ratio to that for a lean mixture. In the engine disclosed in Japanese Unexamined Patent Publication No. 63-129140, when a rate of change in the pressure in the intake passage becomes smaller than a fixed negative value after the acceleration of the engine is completed, the air-fuel mixture is changed from that for the stoichiometric air-fuel ratio to that for a lean mixture.
The output torque of an engine operated by using the lean air-fuel mixture is much smaller than that where the engine is operated by using the mixture for the stoichiometric air-fuel ratio, and therefore, when the air-fuel mixture is changed from the mixture for the stoichiometric air-fuel ratio to the lean mixture after the acceleration of the engine is completed, the output torque of the engine is abruptly lowered. In addition, when the operating state of the engine is changed from the acceleration state to the cruising state, since the throttle valve is slightly closed, the amount of air fed into the engine cylinders is reduced, and thus a decelerating force acts on the vehicle. If the air-fuel mixture is changed from the mixture for the stoichiometric air-fuel ratio to the lean air-fuel mixture when the amount of air fed into the engine cylinders is reduced as mentioned above, the reduction in the output torque of the engine is superimposed on the decelerating force due to the reduction in the amount of air fed into the engine cylinders, and thus there is no danger that the driver will be made uncomfortable by an abrupt reduction in the output torque. Nevertheless, in the above-mentioned known engines, since it is impossible to successfully detect only the change from the acceleration state to the cruising state, or since a time at which the decelerating force due to the reduction in the amount of air fed into the engine cylinders occurs deviates from a time at which the reduction in the output torque of the engine occurs, even if the change from the acceleration state to the cruising state can be successfully detected, the driver will be made uncomfortable by an abrupt reduction in the output torque.
Next, this phenomenon will be described with reference to FIG. 6.
FIG. 6 illustrates the state wherein the degree of opening TA of the throttle valve is slightly reduced when the acceleration of the engine is completed. When a short time has elapsed after a reduction in the degree of opening TA of the throttle valve has begun, a reduction in the amount of air fed into the engine cylinders is begun, and as a result, the acceleration G becomes negative, i.e., a decelerating force acts on the vehicle. In FIG. 6, the solid line illustrates the case wherein the air-fuel mixture is changed from the mixture for the stoichiometric air-fuel ratio to the lean mixture when a ratio of change .DELTA.TA of the degree of opening TA of the throttle valve becomes smaller than a fixed negative valve TAO, as disclosed in Japanese Unexamined Patent Publication No. 60-13936, a one-dot and dash line illustrates the case wherein the air-fuel mixture is changed from the mixture for the stoichiometric air-fuel ratio to the lean mixture when a ratio of change .DELTA.PM of the pressure PM in the intake passage becomes smaller than a fixed negative value PMO, as disclosed in Japanese Unexamined Patent Publication No. 63-129140. Nevertheless, since the rate of change .DELTA.TA of the degree of opening TA of the throttle valve and the rate of change .DELTA.PM of the pressure PM is small, it is difficult to correctly detect that the .DELTA.TA has become smaller than the fixed value TAO or that the .DELTA.PM has become smaller than the fixed value PMO. As a result, a problem arises in that, when the operating state of the engine is changed from the acceleration state to the cruising state, the air-fuel mixture will not be changed from the mixture for the stoichiometric air-fuel ratio to the lean mixture, and thus the engine will continue to be operated while using the mixture for the stoichiometric air-fuel ratio. Further, if the air-fuel mixture is changed from the mixture for the stoichiometric air-fuel ratio to the lean mixture when the .DELTA.TA becomes smaller than TAO or when the .DELTA.PM becomes smaller than PMO, since the .DELTA.TA has become smaller than TAO before a reduction of the amount of air Q fed into the engine cylinders is begun, the decelerating force G.sub.1 due to the reduction in the output torque of the engine occurs before the decelerating force G.sub.0 due to the reduction in the amount of air Q occurs, and since the .DELTA.PM becomes smaller than PMO after the reduction of the amount of air Q is begun, the decelerating force G.sub.2 due to the reduction in the output torque of the engine occurs after the decelerating force G.sub.0 due to the reduction in the amount of air Q occurs. In this case, since the decelerating force G.sub.0 due to the reduction of the amount of air Q occurs based on an operation by the driver, the occurrence of the decelerating force G.sub.0 will not make the driver feel uncomfortable. Nevertheless, since the decelerating force G.sub.1 or G.sub.2 due to the reduction in the output torque of the engine does not occur based on the operation by the driver, the occurrence of the decelerating force G.sub.1 or G.sub.2 will make the driver feel uncomfortable. Furthermore, since a considerable change occurs in the pressure PM when the engine is rotating, a drawback occurs in that the .DELTA.PM becomes smaller than the fixed value PMO even though the operating state of the engine has not changed.
In FIG. 6, the broken line illustrates the case wherein the change in the pressure PM in the intake passage is blunted, and the air-fuel mixture is changed from the mixture for the stoichiometric air-fuel ratio to the lean mixture when a rate of change .DELTA.PMA of this blunted value PMA of the pressure PM becomes smaller than a fixed negative value PMAO. In this case, since a large amount of change occurs in the .DELTA.PMA, it is possible to correctly detect the change from the acceleration state to the cruising state. Nevertheless, since the blunt value PMA is not changed until a short time after the pressure PM is changed, the decelerating force G.sub.3 due to the reduction of the output torque of the engine occurs a short time after the decelerating force G.sub.0 due to the reduction in the amount of air Q occurs. Consequently, in all of the examples illustrated in FIG. 6, a problem arises in that the driver is made uncomfortable when the operating state of the engine is changed from the acceleration state to the cruising state.